MdDave---Unless the deer is nearly directly in front of me it seems my lower limb is banging against the railing. Maybe I'm just used to hang ons with no railing I don't know. As far as noise is, I know they are part of the game with climbers. My brother in law has a Viper too, and loves it.
YOU have total control over where you place your upper stand portion. Position your seat high enough...and you won't have the rail/limb issues.
Also....think LOGICALLY about what you're saying occurred. How could it "slip"? Did the knuckle come loose from the cable? That's the only way this could happen (other than the metal receiver bending and allowing the knuckle to slide through it).
If you didn't have the knuckle seated correctly....it might still climb fine (due to friction).
Like I said....think about it logically. If the knuckle didn't come loose from the cable...and the receiver didn't bend open and allow it to slide through.....then you didn't have it seated properly before you ascended.